The present invention relates to an alternating current generator having a plurality of sets of armature windings. More particularly, the alternating current generator of the present invention is suitable for use in a vehicle.
There have been used an alternating current (hereinafter referred to as a.c.) generator for a vehicle in which a rotor having field windings is disposed inside a stator having three-phase windings.
In this case, in order to increase the number of poles, a rotor is provided on the outer periphery thereof with a plurality of claw-shaped poles formed on a pair of magnetic cores. The armature of a stator is provided with 3 P slots (grooves) (one slot per phase and per pole) for the number of phases of the windings 3 and the number of poles P. The above-mentioned three-phase windings are disposed in these grooves and a tooth-shaped core is formed between a pair of neighboring slots or grooves.
However, since the a.c. generator for a vehicle having the claw poles has one set of three-phase winding and 3P slots (namely, one slot per phase and per pole), a magnetic flux 104 formed between poles 101 and 102 leaks through a tooth-shaped core 103 between two windings A as shown in FIG. 9a, when the poles 101, 102 of the rotor and the tooth-shaped core 103 of the armature take a relative position as shown in FIG. 8.
Such leakage of magnetic flux not only reduces the effective magnetic flux which contributes to electric power generation, but also generates pulsating magnetic flux.
Accordingly, this causes a magnetic noise to be increased, a generated voltage to fluctuate and an output waveform to be distorted, giving rise to ripples when a generated output is rectified into a direct current (hereinafter referred to as d.c.).
It has been demanded that such a magnetic noise is reduced all the more in a power source for a vehicle, etc. which power source feeds various electrical equipment utilizing electronic components and ICs. Further, a high quality direct current power source having less noise and voltage fluctuation due to a ripple, etc. is demanded.
It is an object of the present invention is provide an a.c. generator, in which a magnetic noise is reduced and which generates a d.c. rectified output having a stabilized voltage, by reducing leakage magnetic flux occurring in an a.c. generator.
The inventors have found that it is useful to increase magnetic resistance of a magnetic path for the leakage magnetic flux 104 in order to decrease the leakage of magnetic flux and further that it is useful to split the tooth-shaped core 103 between the slots as shown in FIG. 9b. The inventors have found further that the leaked magnetic flux 104xe2x80x2 can be utilized as effective magnetic flux interlinked with windings by inserting separate armature windings Axe2x80x2 in the slots formed in the split tooth-shaped cores as shown in FIG. 9c and that a resultant rectified output can be stabilized.
Based on the findings, there is provided an alternating current generator comprising a rotatably supported field rotor having a pair of opposed rotor pole cores, each being provided with P/2 claw poles wherein P is an even number; an armature core located around the outer periphery of the field rotor and having axially extending 3nP slots wherein n is an integer more than one; n independent sets of three-phase windings, each being wound on the armature core by being inserted in the slots so that the n sets of three-phase windings are shifted from each other by electrical angle of xcfx80/(3n) radians; and three-phase rectifiers connected with the n sets of three-phase windings to rectify output voltages generated by the three-phase windings.
In a preferred embodiment of the present invention, each of the claw poles has a trapezoidal peripheral shape and P=12 and n=2 hold, so that the armature core has 72 slots and has first and second sets of three-phase windings wound thereon.
In accordance with the present invention, less magnetic flux flows between adjacent field poles through the same tooth-shaped core, and a period of time during which magnetic flux leaks through the tooth-shaped core is shortened. Accordingly, an amount of reduction in effective magnetic flux for the windings of a stator caused by the leakage magnetic flux decreases, which makes pulsation of magnetic flux difficult to occur. Hence, the fluctuation of a generated voltage and the distortion of an output waveform are reduced, resulting in stabilization of a d.c. rectified voltage.
It is another object of the present invention to reduce a windage noise generated by coil ends of armature windings and also to suppress excessively close assembly of the coil ends.
To accomplish this object, in accordance with the present invention, first and second sets of three-phase windings are inserted in odd and even numbered slots of the armature core, respectively, and center portions of the coil ends of each planes of the first and second sets of three-phase windings projecting on both sides of the armature core are arranged with a pitch pattern of 1, 2, 2, 3, 2, 2 time as large as a unit pitch in a peripheral direction of the armature core.
In such an arrangement, excessive congregation of the coil ends can be prevented so that necessity of forced bending and radial or axial overhang of the coil ends for avoiding such excessive congregation of the coil ends can be decreased. Since the dispersion of the coil ends in a peripheral direction is uniformed, the level of a noise (a fan noise) at a given audio frequency can be reduced. Since the coil ends are disposed with unequal pitches in a peripheral direction of the armature core, the frequency spectrum of the fan noise becomes broader as compared with a case in which the coil ends are arranged with an equal pitch, and, since the auditory sensitivity at the reference frequency is shifted to a lower frequency region, it becomes possible to reduce the level of an acoustic noise.
It is a further object of the present invention to suppress a magnetic noise and also to reduce the ripple content of a direct current produced by rectifying an a.c. voltage.
In order to serve this purpose, both sets of three-phase windings inserted in the above-mentioned 3nP slots are connected in Y-form, each set of three-phase windings is connected with each independent three-phase rectifier, and d.c. output terminals of respective three-phase rectifiers are connected in parallel with each other.
With the above-described configuration, it is possible to effectively reduce leakage magnetic flux and also to reduce the ripple content of a d.c. output by virtue of a phase shift between the output voltage of respective sets of three-phase windings by a predetermined amount.
It is a still further object of the present invention to reduce an electromagnetic noise and the number of rectifiers by shifting the distribution of a resultant magnetomotive force of each phase of the armature windings with the rotational movement of the rotor.
In order to accomplish this object, two sets of three-phase windings, including a set of Y-connected windings and a set of delta-connected windings, are used. The number of turns of the delta-connected windings is 1.5 to 2.5 times as many as that of the Y-connected windings. The two sets of three-phase windings are connected in parallel with each other and are connected to a common three-phase rectifier.
With such an arrangement, the distribution of a resultant magnetomotive force of each phase of the armature windings is shifted maintaining a non-varying shape, so that no large pulsating vibrational force is generated between the rotor and the stator. Therefore, an electromagnetic noise can be reduced, without giving rise to degradation of output performance and a rise in cost and in size of a product. Furthermore, no unbalanced circulating current is generated between the two sets of three-phase windings, so that both sets of three-phase windings can be connected in parallel with each other and further can be connected to a common three-phase rectifier.